Perhaps I am not understanding the question completely, but - can't you just specify multiple AD servers for a passthru auth chain? This is what we do for CIFS auth, but - you don't say _what_ you are authenticating (e.g. web, webdav, cifs, ftp).
passthru.authentication.servers=192.168.1.1,192.168.5.100,192.168.10.54
#Offline server check interval in seconds
passthru.authentication.offlineCheckInterval=300